Gijs wrote in Sat Jul 07, 2012 11:20 am:Hi Andrew,
sounds like an interesting project! I've tried to answer your questions:
1) The detailed models usually have some references inside the FDM files (in
Aircraft/p51d/p51d-jsbsim.xml for example, you'll find some comments like "from NACA P-51X and P-51B test data" or "The rest of the drag functions are guesses."). JSBSim supports some "reference/documentation tags", but it isn't used very often, in the FlightGear aircraft at least. But the comments as the P51D has them work basically the same. The P51D has one of the better FDMs btw.
...
Cheers,
Gijs
Since the OP asked about how the models are built and where the data comes from I can provide some details with respect to the P-51D JSBSim FDM model. As the comments quoted above indicate some of the data comes from actual aircraft test data from NACA (mostly) and from other official sources such as the USAAF and the Royal Air Force. Also included in this is data gleamed from pilots manuals and training materials from that era (fuel consumption is one example).
The NACA data is contained in perhaps 8 to 10 reports that tested a wide range things like landing characteristics, stall characteristics, rudder control characteristics, high speed dives, rate of climb, speeds at various altitudes and power settings and so on. Also included in this are actual wing section lift/drag wind tunnel test data.
The P-51 series is somewhat unusual in that at the time it was by far the most tested aircraft in existence and various organizations such as NACA generated reams of data for the aircraft which were compiled into various test reports. Much of this is freely available on the NASA data server and is in the public domain. The test data is contained in about 200 pages of reports.
Even with this extensive data set there is much about the aircraft that is not well documented. I will give two examples:
1. The P-51D used NACA derived exhaust nozzles that generated thrust much the same way a rockets does (IE. the high velocity exhaust gases act like a rocket/jet). There is some test data on these nozzles available from NACA but tests used a Spitfire V (earlier less powerful model of the Merlin engine) or a P-40 (Allison engine) and a lot of reading between the lines was needed to model this on the P-51D.
2. The P-51 series is somewhat unique in that it was perhaps the only production aircraft to successfully apply the "Meredith Effect" which uses the heat from the cooling system to generate jet thrust. But there is no test data for this available, because wind tunnels of the needed size and velocity did not exist, other than some data points published in an artificial written by the chief designer, Lee Atwood see
http://aviationclub.aero/article/view/3756/, in the late 1980s where he credits the "Meredith Effect" with much of the aircraft performance and a late 1930s NACA paper of a wind tunnel test using an electrically heated test device to simulate the "Meredith Effect" which was more or less a proof of concept (IE. was it possible - answer YES).
At best the modeling of these two things is more or less an educated guess that gives us an approximation of how these affected the aircraft thrust/drag. This is also true for some of the other "data" used in the FDM. In reality how well a particular aircraft is modeled is a function of two things.
1. The quality of the data available. In some cases like the P-51 there is extensive data available in other cases very little actual test data can be found.
2. How hard the person doing the modeling works on making the FDM as correct as possible. Not much more to add to this other than that this depends on #1 above and if the quality/quantity of data is lacking there is only so much that can be done to create an accurate FDM model.